A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal. These odd multiples fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 kHz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel. The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238* 10.sup.6 symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. There are 832 symbols per data segment. So, with the symbol rate being 10.76 MHz, each data segment is of 77.3 microseconds duration. Each segment of data begins with a data-segment-synchronization (DSS) signal that is a code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The initial line of each data field includes a data-field-synchronization (DFS) signal that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-random noise sequence (or "PN-sequence") followed by three 63-sample PN sequences. The middle one of these 63-sample PN sequences is transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other. The other two 63-sample PN sequences and the 511-sample PN sequence are transmitted in accordance with the same logic convention in all data fields.
The remaining lines of each data field contain data that have been Reed-Solomon forward error-correction coded. In over-the-air broadcasting the error-correction coded data are then trellis coded using twelve interleaved trellis codes, each a 2/3 rate punctured trellis code. Trellis coding results are parsed into three-bit groups for over-the-air transmission in eight-level symbol coding having a one-dimensional-constellation, which transmission is made without symbol pre-coding separate from the trellis coding procedure. Trellis coding is not used in cable casting proposed in the ATSC standard. The error-correction coded data are parsed into four-bit groups for transmission as sixteen-level symbol coding having a one-dimensional-constellation, which transmissions are made without pre-coding.
The carrier frequency of a VSB DTV signal is 310 kHz above the lower limit frequency of the TV channel. The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed. The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 3-bit symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
VSB signals using 8-level symbol coding is used in over-the-air broadcasting within the United States, and VSB signals using 16-level symbol coding can be used in over-the-air narrow casting systems or in cable-casting systems. However, over-the-air narrow casting systems and cable-casting systems are more likely to use suppressed-carrier quadrature amplitude modulation (QAM) signals. The QAM signals can use 16-state, 32-state or 64-state two-dimensional symbol coding. This presents television receiver designers with the challenge of designing receivers that are capable of receiving either VSB or QAM transmission and of automatically selecting suitable receiving apparatus for the type of transmission currently being received.
This specification assumes that the data format supplied for symbol encoding is the same in transmitters for the VSB DTV signals and in transmitters for QAM DTV signals using two-dimensional symbol coding with a 64-point constellation. The VSB DTV signals modulate the amplitude of only one phase of the carrier at a symbol rate of 10.76* 10.sup.6 symbols per second to provide a real signal unaccompanied by an imaginary signal, which real signal fits within a 6 MHz band because of its VSB nature with carrier near edge of band. Accordingly, the QAM DTV signals, which modulate two orthogonal phases of the carrier to provide a complex signal comprising a real signal and an imaginary signal as components thereof, are designed to have a symbol rate of 5.38* 10.sup.6 symbols per second. This complex signal fits within a 6 MHz band because of its QAM nature with carrier at middle of band. The PN sequences which appear in the initial data segment of each data field as transmitted in the VSB signal and supplied for symbol decoding currently do not appear in the initial data segment of each data field as transmitted in the QAM signal or supplied for symbol decoding. This is because in the QAM DTV signal the symbols code 6-bit groups of data in a 64-point constellation having two orthogonal dimensions, rather than 3-bit groups of data being symbol coded in one dimension as done in VSB DTV signal.
The transmission of the same data in QAM as decoded from data field synchronizing symbol codes in VSB provides a prescribed data sequence that allows for symbol synchronization during QAM reception without need for resorting to differential encoding. This reduces the likelihood of running data errors, so the error-correcting capabilities of the trellis and Reed-Solomon error-correction codes can be devoted more to the correction of errors caused by noise. Also, there is better compatibility of software, such as digital video electromagnetic tape recordings, for QAM and VSB transmissions.
QAM/VSB DTV signal receivers can be of a type in which the final intermediate-frequency signal is digitized, and synchronize procedures to obtain baseband samples are carried out in the digital regime. A tuner within the receiver includes elements for selecting one of channels at different locations in a frequency band used for transmitting DTV signals, a succession of mixers for performing a plural conversion of signal received in the selected channel to a final intermediate-frequency (I-F) signal, a respective frequency-selective amplifier between each earlier one of the mixers in that succession and each next one of said mixers in that succession, and a respective local oscillator for supplying oscillations to each of the mixers. Each of these local oscillators supplies respective oscillations of substantially the same frequency irrespective of whether the selected DTV signal is a QAM signal or is a VSB signal. The final I-F signal is digitized, and thereafter there are differences in signal processing depending on whether the selected DTV signal is a QAM signal or is a VSB signal. These differences are accommodated in digital circuitry including QAM synchronizing circuitry and VSB synchronizing circuitry. The QAM synchronizing circuitry generates real and imaginary sample streams of interleaved QAM symbol code, by synchronizing the digitized final I-F signal to baseband providing it is a QAM signal and otherwise processing the digitized final I-F signal as if it were a QAM signal to be synchrodyned to baseband. The VSB synchronizing circuitry generates a real sample stream of interleaved VSB symbol code, by synchronizing the digitized final I-F signal to baseband providing it is a VSB signal and otherwise processing the digitized final I-F signal as if it were a VSB signal to be synchrodyned to baseband. A detector is employed to determine whether or not the final I-F signal is a VSB signal to generate a control signal, which is in a first condition when the final I-F signal apparently is not a VSB signal and is in a second condition when the final I-F signal apparently is a VSB signal. Responsive to the control signal being in its first condition, the radio receiver is automatically switched to operate in a QAM signal reception mode; and responsive to the control signal being in its second condition, the radio receiver is automatically switched to operate in a VSB signal reception mode.
QAM/VSB DTV signal receivers of this type as thusfar known have used an equalizer that performs complex equalization. During VSB DTV signal reception complex equalization is performed on the real sample stream of VSB symbol code and either on the imaginary sample stream of VSB symbol code or on an imaginary sample stream of null samples. The sampling rate through the equalizer has to be at least equal to the 10.76* 10.sup.6 symbol per second symbol rate in order adequately to de-ghost and equalize the real sample stream of VSB symbol code. During QAM DTV signal reception complex equalization is performed on the real and imaginary sample streams of QAM symbol code at the same sampling rate as the complex equalization is performed on the real sample stream of VSB symbol code. However, since the QAM signal symbol rate is only 5.38* 10.sup.6 symbols per second, the complex equalizer is over-designed for the QAM symbol code (e. g., having twice as many filter taps as otherwise necessary if the equalizer uses a finite-impulse-response digital filter). The complex equalizer is also over-designed for the VSB symbol code since the imaginary sample stream of VSB symbol code is not required for equalization.
Symbol phase detection to implement symbol synchronization has been done after the equalizer, rather than before, in order to avoid error in determining symbol phase owing to multi-path distortion of the baseband symbol code. When the symbol phase detection procedures for symbol clock synchronization are performed after the equalizer, rather than before the, the over-design problems with the equalizer become more significant because higher sampling rate is required in the equalizer response. In order for the symbol phase detection procedures to be carried out properly sampling must be at a rate higher than the symbol rate, which symbol rate suffices for transmitting symbol information where an absolute constraint on symbol phase has already been imposed. So, in order that symbol phase detection procedures employed for synchronizing the symbol clock with VSB baseband symbol coding can be carried out properly following equalization, the sampling rate through the equalizer has been made 21.52* 10.sup.6 samples per second in prior art designs. However, a sampling rate of only 1.5 times, rather than twice, VSB symbol rate secures sufficient bandwidth in excess of Nyquist bandwidth to synchronize the symbol clock with VSB baseband symbol coding. A sampling rate only a few percent larger than VSB symbol rate suffices to provide the excessive bandwidth needed for quick synchronization. A lower sampling rate through an adaptive equalizer reduces the number of multipliers required for adjusting the weights the equalizer uses to implement weighted addition, reducing the cost of constructing the equalizer.